It is known to make composite materials constituted by a reinforcing element (for example fiber) and a matrix in which said reinforcing element is embedded. It is also known that the properties of the composite materials obtained depend a great deal not only on the nature and the properties of the materials which compose them but also on the possibilities of catching (interfacial properties) between the matrix and the reinforcing element. A certain amount of research has therefore been directed towards the modification of the superficial properties of the reinforcing element in order to render it compatible (or catching better) with the matrix.
Within the scope of such research, a process has already been described in which the reinforcing elements (fibers) were subjected, by passage between two electrodes, to electrostatic fields produced by the use of direct and/or alternating electric currents under high voltage. It has been indicated that the electrostatic field produced from a direct current essentially caused a swelling of the reinforcing elements and that the electrostatic field produced from an alternating current caused an etching of the surface of the reinforcing elements and possibly a partial oxidation of said surface. It will be recalled that the electric currents used--the same as those employed in the present invention--have, for the direct currents, a voltage of 50,000 and 150,000 V and, for the alternating currents, a frequency of between 50 and 1,000 Hz (preferably between 200 and 500 Hz) and a voltage of between 10,000 and 30,000 V. These properties of the electrostatic fields bring about a modification of the catching between the reinforcing element and the matrix and consequently a modification (generally an improvement) of the properties of the composite material obtained.